1. Technical Field
The present invention relates to a magnetic recording apparatus, magnetic recording medium, and magnetic recording method.
2. Background Art
For achieving ever-higher densities in magnetic disc apparatuses, it is considered advantageous to use a double-layer perpendicular magnetic recording system. This system employs a double-layer perpendicular magnetic recording medium having a soft magnetic underlayer, and a single pole type (SPT) head having a main pole and an auxiliary pole. However, magnetic disc apparatuses of the double-layer perpendicular magnetic recording system have a problem in that, when the track pitch and the track width of the SPT head are narrowed to achieve higher recording density, residual magnetization remains at the tip of the main pole of the SPT head even when the recording current is made zero.
FIGS. 13A and 13B schematically show how residual magnetization remains at the tip of the main pole of a SPT head. FIG. 13A is an example where a track width Tw is wide, and FIG. 13B shows an example with a narrow track width Tw. When the track width is wide, the magnetization at the tip of the main pole can take a magnetization state such that the magnetic flux is confined within the pole and that no magnetic field escapes to the outside, as indicated by arrows in FIG. 13A. However, when the track width is narrow, the pole tip has a small and long shape, as shown in FIG. 13B, so that the magnetization can be easily directed in a perpendicular direction, resulting in residual magnetization. Since the double-layer perpendicular magnetic recording system employs a soft magnetic underlayer disposed opposite the main pole, the magnetization at the tip of the main pole tends to be more readily directed in a perpendicular direction.
When there is residual magnetization at the tip of the main pole, the flux leaking therefrom tends to flow toward the soft magnetic underlayer, and the resultant magnetic field could erase the data (recording magnetization) recorded in the recording layer or reduce the recording magnetization (resulting in a reduction of reproduction output, which will be hereafter referred to as a “post-recording erasure phenomena”). To overcome this problem, research has been conducted in the prior art to devise an SPT head in which the tendency of residual magnetization to remain is reduced by incorporating certain features into the structure or material of the head.
On a magnetic disc, servo areas are radially disposed in which position information is recorded. Currently, about 50 to 100 servo areas per track are provided. Tracks are provided on the magnetic disc in a concentric manner, with each track divided into a servo area 71 and a data area 72 where user data is recorded. As shown in FIG. 14, the data area is divided into sector blocks 73, so that user data can be recorded on a sector by sector basis. As shown in an enlarged view at the bottom of FIG. 14, each of the sector blocks 73 includes a preamble portion 81 for the extraction of sampling phase, for example, a sync portion 82 indicating the start of user data, a user data portion 83, an ECC portion 84 for a user data error correction code, and a postamble portion 85 whose main purpose is to converge metric calculations in a maximum likelihood decoder for the decoding of data.
As the length of the sector is different from the interval of the servo areas, a sector could overlie on a servo area. If that happens, the sector is split to divide user data in the middle, as shown in FIG. 15. Then, as shown in FIG. 16, a postamble portion 85 is added at the end of a user-data first-half portion 83a and, after the end of the sector area, a preamble portion 81 and a sync portion 82 are newly provided. User data 83b for the latter half is then recorded, an ECC portion 84 is added at the end, and a postamble portion 85 is added, thus completing one sector.
The recording pattern in the sector blocks follows a clock cycle determined for each disc radial position, as shown in FIG. 17, the pattern consisting of a combination of patterns of bits that are integer multiples of a minimum bit length T. The postamble portion, which has the purpose of converging metric calculations in a maximum likelihood decoder, normally consists of a repetition of bits that are twice the minimum bit length, or an arbitrary pattern suitable for convergence. However, in the case of the repetition pattern of minimum bit length, the input to the maximum likelihood decoder becomes zero so that the metric calculations cannot be converged. Accordingly, the minimum-bit length repetition pattern is not conventionally used in the postamble.
(Non-Patent Document 1)
W. Cain, IEEE Trans. Magn., 32, 97 (1996)